Both AC and DC powered solenoids are common in the prior art and are used in a wide variety of applications requiring small control movement such as electrical contactors or fluid control valves. Either an AC or DC power supply is used to create an electromagnetic field in a core which is usually fabricated from a plurality of laminations of soft iron. The armature is attracted to the magnetic core (pole member) and provides the input force to the device that is to be controlled. The use of an AC electrical current is desirable from the standpoint that a high activation pull-in force is generated in the armature. However, upon contact with the core, the current draw of the AC winding naturally decreases because of the increased inductance. Also, the zero crossing point of the AC flux results in a high noise level due to the high frequency "buzzing" produced by the solenoid. The buzzing noise is caused by the cyclic nature of the AC current starting at zero rising to a maximum positive value and then falling through zero reaching a maximum negative value. For example, if common 60 Hz AC current is used, the buzzing would occur at this frequency due to the force reversals. Common problems with this type of AC induced noise occurs in household and office equipment and in specialized applications such as high power electrical contactors used in military equipment.
Using a DC electrical supply will solve the noise problem of the AC supply, but the level of pull-in force is dramatically reduced at a given level of supply voltage and the temperature rise is dramatically increased. Direct current (DC) solenoids do not exhibit the aforementioned buzzing noise when their motion producing elements are energized into their pull-in position. It is known to those skilled in the art that an AC energized solenoid has more pulling force at a given power level than a DC powered solenoid. Thus, if DC power is to be used, the solenoid must be considerably larger or conversely, higher DC currents must be used to generate the same pulling force as an AC coil which can result in excessive operating temperatures. Conversely, it is known that for a given size, a DC solenoid has a higher holding force at full travel than an AC solenoid along with a much lower noise level due to the non-reversing nature of the DC source.
Solenoids, such as those disclosed in U.S. Pat. Nos. 4,197,444, 4,520,332, and 3,671,899, the disclosures of which are hereby expressly incorporated by reference, use electrically energized coils to produce magnetic fields which act on a rotary or linear element to provide mechanical motion. U.S. Pat. No. 4,544,987, the disclosure of which is hereby expressly incorporated by reference, describes a magnetic switching device which uses an AC power source for energizing for activation of a switching device and subsequentially a DC current for holding the switching device in an activated position.
The referenced prior art discloses methods of using an AC supply for pull-in and then switching at the appropriate time as the armature nears the core to a DC supply generated by rectifying the AC supply. Thus, at any point in time the solenoid is powered by either AC or DC. The problem with this approach is that complicated electrical circuits must be used to both rectify the AC into DC and to perform the switching function at just the right instant of time depending on the position of the solenoid armature which requires the use of a position sensor or complicated logic contained within a smart controller. None of these devices use simultaneous continuous energization of both the AC and DC current sources for operation of the solenoid.